This could happen if either adware/malware/possibly unwanted app is installed or a Profile is added to System Preferences.
1. Remove any profiles you have not installed knowingly.
Profiles
System Preferences > Profiles
Launch System Preferences. Do you see any profiles added to the pane at the bottom?
How to remove a configuration profile? https://support.apple.com/kb/PH25680?locale=en_US
2. Remove Adware/Malware
Please take all these steps.
1. Use MalwareBytes for Mac to remove malware/adware.
Install guide: https://support.malwarebytes.com/docs/DOC-1817
Uninstall Guide: https://support.malwarebytes.com/docs/DOC-1190
The installer may ask you to allow it in Security & Privacy /System Preference. Allow it.
2. Disable all extensions and test.
Disable or uninstall extensions.
Section: Manage extensions: https://support.apple.com/guide/safari/use-safari-extensions-sfri32508/mac
3. Choose a search engine.
Delete all text in the Smart Search field, click the magnifying glass , then choose a search engine from the list.
https://support.apple.com/guide/safari/customize-your-search-ibrwe75c2a3c/mac
4. Set your Home Page. https://support.apple.com/guide/safari/set-your-homepage-ibrw1020/mac
5. Restart your Mac.
6. Launch Safari holding the Shift key down.